1. Technical Field
The present invention is directed to systems that use charged particle beams to draw or write patterns directly onto substrates such as, for example, semiconductor wafers coated with an energy sensitive material.
2. Art Background
In processes that use a charged particle beam to draw or write patterns on substrates, the orientation of the beam relative to the orientation of the substrate is important to ensure not only that the desired pattern is introduced into the substrate, but that each point of the pattern is oriented properly relative to the other points in the pattern being drawn or written and to existing patterns already formed in the substrate. Typically, alignment marks are placed on the substrate, and the beam is directed to write the pattern relative to the alignment marks. The substrate is placed on a mechanically movable stage that adjusts the position of the substrate relative to the beam. The beam is targeted by detecting the alignment marks, and moving the mechanical stage so that the substrate and the beam coincide at the desired point.
Referring to FIG. 1, as noted in U.S. Pat. No. 5,811,026 to Saitou et al., if the orientation flat 21 of the wafer 11 is rotated relative to the stage orientation, which is represented by the directional arrows x and y, then typically the wafer is rotated to properly align the arrangement of the patterns illustrated as chips 24 relative to the image carried by the charged particle beam. The wafer is typically rotated by rotating a mechanical stage on which the wafer is placed. The position of the wafer is adjusted by detecting the orientation of the marks 22 and 28 and making the necessary adjustment by rotating the substrate to obtain the proper orientation of the beam relative to the rotation marks. Saitou et al. proposes that the mechanical stage only be used for translational movement of the wafer (i.e. movement in the x, y and z directions) and that a lens or coil be used to rotate the particle beam in relation to the wafer rather than vice-versa. Saitou et al. notes that it is advantageous to rotate the shaped particle beam itself, because, if the orientation flat 21 is rotated by an angle .theta. relative to the moving direction of the mechanical stage, the line drawn by the beam tends to be stepwise, as illustrated in FIG. 2. As noted by Saitou et al., .theta. should be less than 1 mrad in order to avoid adverse lithographic consequences from the stepwise pattern illustrated in FIG. 2.
Because it is difficult to obtain mechanical prealignment with this degree of precision, Saitou et al. suggests using a rotating lens (e.g. a solenoid coil) to actually rotate the beam relative to the wafer. Saitou et al. notes that this solution is limited because the solenoid coil also introduces magnification or demagnification effects into the beam. Saitou et al. observes that the amount of rotation provided by the coil is limited to the amount that can be provided with substantially almost no beam blurring. Since, in many instances, greater rotation than what can be provided without negligible magnification change is needed, such a restriction provides design constraints on the coil which make it unadaptable to a variety of process conditions. Since a mechanism for rotating a particle beam which adapts to different process conditions is desired, a different solution other than the one proposed by Saitou et al. is sought.